1. Field of the Invention
The present invention relates generally to an optical transmitter and, in particular, to an optical transmitter for generating return-to-zero alternative-mark-inversion (RZ-AMI) optical signals using a Mach-Zehnder modulator.
2. Description of the Related Art
It is generally known in the state of the art that a return-to-zero (hereinafter, referred to as “RZ”) optical signal is adapted to carry any information in its intensity, so that upon representation of bit “1” an energy state of the optical signal moves from “0” energy level to “1” energy level and then returns to “0” energy level. Such return-to-zero characteristics will cause an RZ optical signal to have narrow pulse width. In application of an optical transmission system with data rate of more than 20 Gbps, it may become less sensitive to the non-linearity of an optical fiber that serves as a transmission medium for the optical signal.
Although the intensity of optical signal in such a return-to-zero alternative-mark-inversion (hereinafter, referred to as “RZ-AMI”) modulation system may be substantially same as that in RZ optical signals, the RZ-AMT modulation system is characterized in that the phases of optical signals are inverted for every “1” bit. Therefore, while the RZ-AMI optical signal is designed to keep the advantages of RZ modulation system, its narrow spectrum width gives another advantage in improving the frequency efficiency in a dense wavelength division multiplexed (DWDM) optical transmission system, and the optical signal is generally less sensitive to dispersion of optical fiber. Moreover, as the phases of the RZ-AMI optical signal are inverted for every “1” bit, its carrier-frequency components may be suppressed to allow more resistance to Brillouin non-linear effect.
FIG. 1 shows a schematic block diagram of a conventional RZ-AMI optical transmitter 100 including a precoder 110, a low-pass filter (LPF) 120, a continuous wave laser diode (CW-LD) 130, and first and second Mach-Zehnder modulators (MZM) 140 and 150. The precoder 110 further includes a 1-bit delay circuit (T) 114 and an exclusive-OR circuit 112 for precoding binary data of non-return-to-zero (NRZ) signal as inputted. The low-pass filter 120 serves to limit the bandwidth of the precoded data. This low-pass filter has 3 dB bandwidth corresponding to one quarter of the data rate and precodes the input precoded data to ternary data with its limited bandwidth. For example, when the data rate is 40-Gbps, the low-pass filter 120 may have a 10-GHz, 3-dB bandwidth. The continuous wave laser diode 130 provides at its output a CW mode of light beam.
In the meantime, the first Mach-Zehnder modulator 140 functions to modulate an incident light from the continuous wave laser diode 130 on basis of the ternary data to generate a duobinary optical signal at its output. Here, a bias position of the first Mach-Zehnder modulator 140 may be preferably set to a null point corresponding to a minimum value on a transfer curve. The second Mach-Zehnder modulator 150 operates to modulate the duobinary optical signal input from the first Mach-Zehnder modulator 140 on basis of a sine wave clock signal having a frequency corresponding to one half of a clock frequency of the binary data, for generation of RZ-AMI optical signal at its output. For example, when the data rate is 40 Gbps, the sine wave clock signal has a frequency of 20 GHz, wherein a bias position of the second Mach-Zehnder modulator 150 may be preferably set to a null point corresponding to a minimum value on a transfer characteristic curve. As is in RZ signals, in representation of “1” bit, the RZ-AMI optical signal allows its energy to move from “0” energy level to “1” energy level and then come back to “0” energy level, while inverting its phase either from “0” to “π” or from “π” to “0” for every “1” bit.
The RZ-AMI optical transmitter 100 may be constructed with a combination of the typical duobinary optical transmitter using the first Mach-Zehnder modulator 140 and the second Mach-Zehnder modulator 150 for generating carrier-suppressed return-to-zero (CS-RZ) optical signals, so it may be also referred to as a duobinary carrier-suppressed return-to-zero (DCS-RZ) optical transmitter.
FIG. 2 shows an eye diagram of an RZ-AMI optical signal output from the optical transmitter 100 shown in FIG. 1. As seen in FIG. 1, it will be appreciated that ripples 210 are formed on a space (“0” or “low”) level of the eye diagram when a low-pass filter 120 is used with 3-dB bandwidth corresponding to one quarter of the binary data rate. These ripples usually result in a deterioration of the receive sensitivity in a receiving end of RZ-AMI optical signal.